Scalable electrosynthesis of commodity chemicals from biomass by suppressing non-Faradaic transformations

Electrooxidation of biomass platforms provides a sustainable route to produce valuable oxygenates, but the practical implementation is hampered by the severe carbon loss stemming from inherent instability of substrates and/or intermediates in alkaline electrolyte, especially under high concentration. Herein, based on the understanding of non-Faradaic degradation, we develop a single-pass continuous flow reactor (SPCFR) system with high ratio of electrode-area/electrolyte-volume, short duration time of substrates in the reactor, and separate feeding of substrate and alkaline solution, thus largely suppressing non-Faradaic degradation. By constructing a nine-stacked-modules SPCFR system, we achieve electrooxidation of glucose-to-formate and 5-hydroxymethylfurfural (HMF)-to-2,5-furandicarboxylic acid (FDCA) with high single-pass conversion efficiency (SPCE; 81.8% and 95.8%, respectively) and high selectivity (formate: 76.5%, FDCA: 96.9%) at high concentrations (formate: 562.8 mM, FDCA: 556.9 mM). Furthermore, we demonstrate continuous and kilogram-scale electrosynthesis of potassium diformate (0.7 kg) from wood and soybean oil, and FDCA (1.17 kg) from HMF. This work highlights the importance of understanding and suppressing non-Faradaic degradation, providing opportunities for scalable biomass upgrading using electrochemical technology.

fixed to be same via one peristaltic pump like in supplementary Figure 36. Is this parameter studied in more detail?
10. How does the gas-liquid separation chamber work in the stacked SPCFR system in supplementary Figure 36? What is the amount and purity of the hydrogen that generated and separated?

Rev #3
I am very positive about the publication of this article. The data presented here is relevant to the field and the conclusions are well-supported by the experiments. Below are some comments.
1) During the oxidation of the biomass-derived molecules, green hydrogen is produced in the cathode. It called my attention that the authors have not mentioned the importance of the production of this energy vector.
2) In this paper, the separation and quantification of the products are extremely relevant. The main drawback of this paper is that the authors did not clearly explain the HPLC protocol used to run the samples. They should also show which products are they able to indeed separate.
3) About the mechanism, you could mention something more than simply "providing experimental evidence for our previous theoretical calculation". I think that the calculations you published are extremely valuable, but the level of detail of them is much deeper than what these results can show. Here you show that you break the C-C bond and the importance and reactivity of the aldehydes, which is really valuable, but you are not giving much evidence for mechanistic details. To link these observations (I mean, the products generated in different conditions) to the detailed mechanism is too much. What indeed can give stronger insights into the mechanism are, for example, Raman in situ experiments like that you performed in the same paper and/or a simpler experiment like that shown in figure 17 of this manuscript: https://chemrxiv.org/engage/chemrxiv/article-details/63f64e9032cd591f12534067 The importance/reactivity of the aldehydes have been shown in several articles but most of them use PGM electrodes. I am not suggesting including the references but maybe they help improving the discussion.
https://www.sciencedirect.com/science/article/pii/S0013468618325581?via%3Dihub 4) Authors achieved the following "By preliminary optimization of current and flow rate, and by temperature management of feeding solution, we achieved high productivity of formate in electrooxidation of glucose, delivering high SPCE (81.8%), good formate selectivity (76.5%), and high FE (91.7%) at current of 15 A (Supplementary Table 4)." Considering 100% of faradaic efficiency, I think informing the green hydrogen productivity is also important. 5) In the Large-scale electrosynthesis experiments, the authors state that the cell working temperature is around 70°C and that they must work on temperature management. I think that instead of cooling down the system, it is maybe better to optimize the method at this relatively high temperature.
6) Again, about the production of value-added molecules. The authors informed the concentration of products. As they know the flow in the cell, they could inform the mass of products obtained per unit of time, for instance, per hour. 7) To close, I would like to suggest the authors stop using Pt CE. The Pt atoms can migrate to the working electrode and deposit there.

Response to Reviewer #1:
In this manuscript, the authors investigated the limitations in process scale-up of biomass platforms electrooxidation and proposed a general and efficient system engineering strategy for the preparative electrosynthesis of valuable products from typical biomass platform chemicals. They show that degradation of instable substrate and reaction intermediates in alkaline electrolyte, especially at high concentration, is one of the major challenges for scaling-up of reaction system of electrocatalytic biomass upgrading. On this foundation, they constructed a single-pass continuous flow reactor system for selective and scalable upgrading of biomass derivatives, such as glucose, HMF. The electrocatalytic conversion of organic substrates (e.g., biomass derivatives) is an active research area nowadays. Unlike most research focusing on efficient catalyst synthesis, the authors demonstrated the importance and opportunities of system engineering for process scalability, and unveiled the reaction pathways that is often misunderstood by non-Faradaic reaction interference. The manuscript is well organized and well supported by a suite of experimental data. I believe this work will have a notable impact for the development of electrocatalytic biomass upgrading towards practical application. Therefore, I recommend its publication in Nature Communications after the authors address the following minor issues:  FDCA (Angew. Chem. Int. Ed. 2016, 55, 9913−9917). As shown in revised Supplementary Fig. 18, FDCA selectivity dramatically decreased when HMF electrolyte with higher concentration (200 mM) and larger volume (50 mL) was used. Specifically, FDCA selectivity decreased from 92.9% (scenario A) and 86.7% (scenario C) to 46.6% (scenario B). Therefore, we can conclude that the similar trend shown in Fig. 2 was observed when well-studied anodic catalyst was used (revised Supplementary Fig. 18), indicating that the carbon loss issue in HMF electrooxidation is not mainly related to the selection of catalyst.

Based on above discussion, we revised the Manuscript and Supplementary
Information as follows: "To demonstrate its general applicability, we further evaluated the HMF oxidation performances with nickel foam supported nickel phosphide (Ni2P/NF, Supplementary   Fig. 17), which was previously demonstrated as a selective catalyst for HMF electrooxidation to FDCA 52 . A similar catalytic trend was observed when replacing CoOOH/NF with Ni2P/NF ( Supplementary Fig. 18), indicating that the carbon loss issue in HMF electrooxidation is not mainly related to the selection of catalyst." High-resolution TEM image of Ni2P nano-particle. c XRD pattern of Ni2P/NF. Figure 17a, SEM images revealed nano-array structure of Ni2P, and the nano-array is composing of interconnected nano-particles (inset). TEM combined with XRD characterizations (Supplementary Fig. 17b,c) further confirmed the Ni2P structure. Figure 18. Catalytic performances of HMF electrooxidation in different scenarios (different feedstock concentrations and electrolyte volumes) at 1.5 V vs RHE using Ni2P/NF as the anode.

Revised Supplementary
As shown in Supplementary Fig. 18, FDCA selectivity dramatically decreased when HMF electrolyte with higher concentration (200 mM) and larger volume (50 mL) was used. Specifically, FDCA selectivity decreased from 92.9% (scenario A) and 86.7% (scenario C) to 46.6% (scenario B). Therefore, we can conclude that the similar trend in Comment 2: Page 7, the authors need explain the sentence "Preliminary kinetic analysis reveal that approximate 54% of glucose was consumed via non-Faradaic reactions during electrolysis (i.e., scenario B at 1.5 V vs RITE), ...".
Response: This is a critical comment. Indeed, the original expression was ambiguous.
In our experiments, we evaluated reaction velocity of glucose conversion in 50 mL 1 M KOH solution with 100 mM glucose under electrochemical conditions (at 1.5 V vs RHE, corresponding to scenario B at 1.5 V vs RHE in the manuscript) or under chemical conditions (without applying bias nor electrocatalyst, to study non-Faradaic reactions).
The results show that, under electrochemical conditions, the apparent reaction velocity of glucose conversion was estimated to be 5.44×10 −6 mol L−1 s−1 ( Supplementary Fig. 11a), which is contributed by both electrocatalytic and non-Faradaic side-reactions. In contrast, under chemical conditions, the glucose reaction velocity was measured to be 1.64×10 −6 mol L −1 s −1 , which is contributed by non-Faradaic side-reactions, namely base-catalyzed glucose degradation ( Supplementary   Fig. 12). Based on the above catalytic results, we estimate that about 54% of the converted glucose fraction was contributed by non-Faradaic side-reactions during electrolysis (in scenario B in the manuscript).

To clearly present these results, we revised the Manuscript and Supplementary
Information as follows: 1. We rewrite the sentence in the revised Manuscript: "Preliminary kinetic analysis reveals that approximate 54% of the converted glucose fraction was consumed by non-Faradaic side-reactions during electrolysis (in scenario B at 1.5 V vs RHE), indicating that the unwanted non-Faradaic degradation even surpasses the targeted electrocatalytic conversion (see details in Supplementary Note 1, ." (Please see Page 7 in the revised Manuscript) 2. We revised Supplementary Note 1 in the Supplementary Information: "Furthermore, the reaction velocity of non-Faradaic reaction (νnF), that is basecatalyzed glucose degradation, was calculated to be 1.64×10 −6 mol L−1 s−1 (Supplementary Fig. 12; in 1 M KOH solution without applying bias nor electrocatalyst). Based on the catalytic results under electrochemical conditions (at 1.5 V vs RHE) and under chemical conditions (without applying bias nor electrocatalyst), we estimate that about 54% of the converted glucose fraction was contributed by non- ii Rapid transformation: A single-module SPCFR ( Supplementary Fig. 19) was employed for electrocatalytic GOR to formate using a mixed electrolyte composing of 1 M KOH and 100 mM glucose at a flow rate of 11.4 mL min −1 . The SPCFR system enables rapid transformation of glucose, affording detectable reaction intermediates for subsequent HPLC analysis. The SPCFR system also shortens the duration time of glucose and intermediates in the reactor, hence diminishing non-Faradaic degradation. In addition, the SPCFR can be operated at different currents (0−7 A) to detect the variation of possible reaction intermediates.
iii Fast neutralization and analysis: The electrolyte at the outlet of SPCFR was collected and immediately quenched (that is, neutralized) by a dilute acid (0.5 M H2SO4) to stabilize the reactive intermediates. The products were then immediately analyzed by HPLC. By doing this way, the reactive intermediates (such as aldehyde intermediates) can be stabilized without being transformed into organic acids by base-catalyzed degradation. Comment 5: Some sentences need to be more concise, such as "a γ-phase CoOOH with array morphology CoOOH was fabricated on nickel foam (CoOOH/NF),..." on Page 6, the second "CoOOH" can be deleted.

Response:
We thank the reviewer for the helpful suggestion. We checked the manuscript and rewrote the following sentences: "By using an electrodeposition method 32 , a γ-phase CoOOH with array morphology was fabricated on a nickel foam matrix (CoOOH/NF), which was characterized with

Response to Reviewer #2:
In this manuscript, the authors developed a simple and efficient single-pass continuous flow reactor (SPCFR) system to suppress non-faradaic degradation of substrate and improve the selectivity in electrooxidation of biomass, such as polyhydroxy compounds to formate, and HMF to FDCA. A series of experimental evidence solidly support the mechanistic insight of aldose route for oxidative C−C bond cleavage of polyhydroxy compounds, rather than the most adopted aldonic acid route. Moreover, the kilogramscale electrosynthesis of KDF and FDCA are fascinating, showing the potential scalable electrosynthesis. The manuscript could be considered to publish on Nature Communications after addressing the following problem.
Comment 1: In this SPCFR system, a good high single-pass conversion efficiency (SPCE) was achieved. If liquid stream goes through the SPCFR system one more time via peristaltic pump, could the conversion efficiency further improve?
Response: We appreciate for this valuable suggestion.
1. Based on this comment, we constructed a two-tandem-modules reaction system for glucose electrooxidation, in which the reaction solution was feed into the first reactor module, and the liquid stream was then feed into the second one (revised Supplementary Fig. 45a).
Under an optimized conditions (i.e., current of 3 A, flow rate of 1.98 mL min - In terms of catalytic performance, electrolyte that passes through multiple modules at a given flow rate is equivalent to that passes through one reactor at a lower flow rate. This is because when multiple modules are adopted, the duration time of electrolyte in the reactor proportionally increases, with same result obtained in one reactor with lower flow rate. Based on this understanding, we anticipate that the reaction in one reactor may deliver similar catalytic trend (higher conversion but lower formate selectivity and FE) if the flow rate is lower than the optimal one, that is, the electrolyte maintained in the reactor was too long, thereby overoxidation of formate to carbonate takes place. The catalytic results in one reactor at different flow rate was shown in Supplementary Fig. 20. The optimal formate selectivity and FE were obtained at 1.98 mL min −1 . To our expectation, when a lower flow rate was applied (1.89 and then 1.68 mL min −1 ), higher glucose conversion but lower formate and FE were observed, in consistent with the above catalytic results when two tandem modules were used to replace one reactor.
2. As the reviewer suggested, using tandem modules would be efficient to improve the productivity and scalability of biomass electrooxidation technology.
According to the above catalytic comparison, it is expected that similar catalytic results (conversion, product selectivity and FE) but with higher flow rate can be achieved in tandem modules compared with that in one reactor.
In contrast, in a single reactor, we obtained similar catalytic performance ( To investigate if the reaction intermediates and/or products generated via anodic oxidation can be further reduced at cathode, we analyzed HPLC chromatograms of glucose-electrolysis product in an undivided cell and also in a H-cell (equipped with AEM). As reported in previous literature (ACS Catal. 2020, 10, 13895−13903), electrochemical reduction of sugars results in polyols at alkaline medium (pH>11). As shown in revised Supplementary Fig. 6c, HPLC analysis indicated that similar products distribution was obtained by using undivided cell and H-cell, without observing associated reduction products (e.g., polyols) from glucose and intermediates in the undivided cell, suggesting that the reaction intermediates and/or products generated via anodic oxidation cannot be further reduced at cathode under our reaction conditions.
Inspired by the reviewer's comment, we consider that if the catalyst at cathode (nickel foam) is active enough for electroreduction, or if the organic substrates and/or products are inevitable to be reduced, an ionic exchange membrane should be present in the reactor to avoid electroreduction.
Based on the above discussion, we have revised the Supplementary Information as following: "In addition, HPLC chromatograms show that similar products distribution was obtained by using undivided cell and H-cell, without observing associated reduction products (e.g., polyols) from glucose and intermediates ( Supplementary Fig. 6c), suggesting that glucose and associated intermediates and products may not be reduced First, we apologize that the mentioned "92.3%" is carbon balance value, which was mistakenly wrote as FDCA selectivity in the original manuscript. The actual FDCA selectivity was measured to be 91.3%. The total selectivity of other detectable products is 0.70%, including DFF (0.13%), HMFCA (0.38%), and FFCA (0.19%), as shown in revised Supplementary Fig. 27.
Second, as the reviewer mentioned, other by-products were expected to generate according to the carbon balance results. We speculate that the carbon loss (that is 7.7%, according to carbon balance of 92.3%) is mainly attributed to the formation of humins, with fewer generation compared with the results in batch reactor, which is challenging to quantify. Response: We appreciate the reviewer for pointing the flow rate-dependent performance, which is very important. We response it from the following two aspects:  Supplementary Fig. 20). Similarly, during HMF electrooxidation to FDCA (the revised Supplementary Fig. 25b), the intermediate HMFCA and FFCA were gradually converted to FDCA as the flow rate decreased, leading to higher FDCA selectivity.
2. Regarding the lower Faradaic efficiency as flow rate decreases, it is attributed to the decrease of substrate and intermediates concentration in the electrolyte at low flow rate. As a result, oxygen evolution reaction become a competitive reaction over anode owing to insufficient mass transfer, resulting in the decrease of Faradaic efficiency during electrooxidation of glucose and HMF at lower flow rate. In addition, overoxidation of formate also contributed to the lower Faradaic efficiency during glucose oxidation. As shown in Supplementary Fig. 20, formate selectivity decreased when flow rate is lower than 1.98 mL min −1 , indicating that formate generation is diminished by the overoxidation.
Based on the above discussion, we have revised the Supplementary Information as following: "As the flow rate decreases, the duration time of substrate and intermediates in the reactor increases, leading to higher conversion of glucose and intermediates (such as arabinose). After optimizing flow rate, we obtained the highest formate yield (67.2%) and selectivity (83.8%) at 1.98 mL min -1 . At lower flow rate, the oxygen evolution and formate overoxidation become more competitive, resulting in the decrease of formate   Fig. 4a). In addition, Co 2p XPS spectra and Raman spectra of the spent CoOOH were well consistent with that of the fresh one (revised Supplementary Fig. 31b, c), confirming the stability of CoOOH/NF anode.

Based on these results, we revised the Manuscript and Supplementary
Information as follows: "No obvious decay of catalytic performances and catalyst structure was observed for more than 100-hours at current of 5 A (Fig. 3f, Supplementary Figs. 30, 31)." (Please see Page 11 in the revised Manuscript) "After stability test (Fig. 3f), the spent CoOOH/NF anode was characterized by SEM, XPS, and Raman techniques. As shown in Supplementary Fig. 31a, the structure of CoOOH/NF was maintained, showing similar nano-array structure to that of the fresh catalyst ( Supplementary Fig. 4a). In addition, Co 2p XPS spectra and Raman spectra of the spent CoOOH were well consistent with that of the fresh one ( Supplementary Fig.   31b, c), confirming the stability of CoOOH/NF anode." (Please see Page 28  Co 2p XPS spectra and (c) Raman spectra of fresh and spent CoOOH. Figure 31 and 32, the integration of NMR spectra is unclear, it is better to provide clearer one.

Response:
We appreciate the reviewer for raising this valuable comment.
Supplementary Figures 31 and 32 have been replotted to improve the readability, as shown below:

Based on these results, we revised the Manuscript and Supplementary
Information as follows: "To optimize the ratio of glucose/KOH, we evaluated the catalytic performances of glucose electrooxidation at a fixed concentration (100 mM) but with different concentration of KOH electrolyte (from 0.5 to 2 M). As shown in Supplementary Fig.   21, inferior catalytic performances (e.g., FE and selectivity of formate is <40%) were obtained in 0.5 M KOH electrolyte. This can be explained by the overoxidation of formate to carbonate, as large quantity of bubbles was generated when the electrolyte was acidified for HPLC analysis. In contrast, good catalytic performances can be achieved (the FE and selectivity of formate is >80%) after KOH concentration was Response: We appreciate the reviewer for raising these insightful comments.
1. To clearly illustrate the working mechanism of the gas-liquid separation chamber, the scheme has been revised. As shown in revised Supplementary Fig. 43, there are two outlets in the gas-liquid separation chamber of the stacked SPCFR, with the upper one for gas stream and the lower one for liquid stream. In addition, there is a gate before the outlets. The gas (that is H2) is flowing through the gate and exiting from the upper outlet because of its low density. The liquid (that is electrolyte) is flowing through the gate, falling down and exiting from the lower outlet. This design for separating gas and liquid is inspired by a previous work from Kato and colleagues (Joule 2021, 5, 687−705), which was originally designed for CO2 electroreduction.
2. In the last version of the manuscript, the generated gas stream was directly pass through a gas analyzer without drying, giving H2 purity of 97.69% (revised Supplementary Fig. 42b), which contains a small fraction of water vapor. To remove water vapor, we connected the gas outlet of the electrolyzer to a drying column with desiccant silica gel before entering the hydrogen analyzer (revised Supplementary Fig. 43). As a result, the H2 purity was determined to >99.9%. In addition, H2 productivity was calculated to be 279.8 mmol h −1 when the reaction was operated at 15 A.

Based on above results and discussion, we revised the Manuscript and
Supplementary Information as follows: "As a result, formate solution (562.8 mM) with space-time-yield (STY) of 256.6 mmol h−1 (corresponding to 11.8 g h −1 ) coupling with H2 production (>99.9% purity) with STY of 279.8 mmol h −1 (corresponding to 0.56 g h −1 ) were continuously produced using this stacked SPCFR system (Supplementary Figs. 43,44), achieving co-production of biomass-derived valuable chemicals and H2 fuel." (Please see Page 15 in the revised

Manuscript)
"There are two outlets in the gas-liquid separation chamber of the stacked SPCFR, with the upper one for gas stream and the lower one for liquid stream. In addition, there is a gate before the outlets. The gas (that is H2) is flowing through the gate and exiting from the upper outlet because of its low density. The liquid (that is formate solution) is flowing through the gate, falling down and exiting from the lower outlet.
This design for separating gas and liquid is inspired by a previous work by Kato and colleagues 37 , in which a reactor was designed for CO2 electroreduction. The generated gas from the electrolyzer passed through a drying column filling with desiccant silica gel before it entered to a H2 analyzer. After drying treatment, H2 purity was measured to be 99.99%, higher than that without drying (97.69%; Supplementary Fig. 42b)." Comment 2: In this paper, the separation and quantification of the products are extremely relevant. The main drawback of this paper is that the authors did not clearly explain the HPLC protocol used to run the samples. They should also show which products are they able to indeed separate.

Response:
We thank the reviewer for raising the important comment.  Catal. 2022, 5, 268−276). In our work, the generated formate (in potassium salt form) was acidified by formic acid to produce potassium diformate (KDF) via decolorization, concentration, crystallization, filtration and drying processes ( Supplementary Fig. 53). KDF is a value-added commodity as a feed additive for promoting animal growth. For the product of HMF electrooxidation, FDCA can be isolated by acidifying and filtration ( Supplementary Fig. 30).
Therefore, both formate and FDCA can be isolated by mature separation units.   1. In our previous theoretical studies on glucose electrooxidation to formate (Angew. Chem. Int. Ed. 2023, e202219048), the calculation results indicate that the C−C bond cleavage is initiated at C1−C2 position to give arabinose and formate, owing to the smallest bond order among the five C−C bonds in glucose (as the figure shown below). To clearly demonstrate the specific coincidence between experimental findings in this work and previous theoretical results, we revised the manuscript as following: "By combining above HPLC results and 13 C1-labeling experiments (see discussion in Supplementary Note 3), we can exclude route II for glucose-to-formate, and rationalize that the reaction was initiated from C1−C2 bond position of glucose ( Fig. 4a (up)). This  First, we performed isotope experiments using 13 C1-labeled glucose ( Supplementary Fig. 37) and gluconate ( Supplementary Fig. 38) as the substrates, respectively. As shown in Supplementary Fig. 37, 13 C-labeled formate was produced when 13 C1-labeled glucose was used as the substrate, indicating that the aldehyde group in glucose was transformed into formate. In addition, the semiquantitative 1 H NMR analysis shows that the ratio of 13 C/ 12 C in the generated formate from 13 C1-labeled glucose reached 0.3 (higher than the theoretical value 0.2) at low charge, and it successively decreased at longer reaction time ( Supplementary Fig. 39). This trend can be explained by that the C1−C2 bond cleavage in glucose is more favourable, thus the 13 C1-labeled aldehyde group is firstly converted into formate. This is in well agreement with our previous theoretical calculation that glucose electrooxidation is initiated at C1−C2 position to give arabinose and formate, owing to the smallest bond order among the five C−C bonds (Angew. Chem. Int. Ed. 2023, e202219048).
In contrast, the ratio of 13 C/ 12 C in the generated formate from 13 C1-labeled gluconate is about 0.01 (equal to the natural abundance of 13 C/ 12 C) in the whole electrolysis process (Supplementary Fig. 39). This result reveals that the acid group of gluconate cannot be converted to formate, thus route II can be excluded. These results further demonstrated that the aldehydes, rather than acids, are the real intermediates of C−C bond cleavage to formate in glucose electrooxidation.
Second, we analyzed the products during electrooxidation of glycolic acid, glyceric acid, lactic acid, and corresponding polyols, to identify if the carboxy group in aldonic acid can be converted to formate. As shown in Supplementary   Fig. 40, excellent carbon balance (90−100%) were obtained when polyols were used as the substrate, much higher than the results when associated acids are used as the substrate (carbon balance of 50−63%). The most possibility is that during electrooxidation of the acid molecule, the carboxy group was converted to CO2 (route III in Fig. 4b). Collectively, we can exclude glycolic acid and glyceric acid as the main intermediates in electrooxidation of glucose, glycerol, and other biomass derived polyhydroxy compounds.
Based on the above results, we have revised Supplementary Note 3 to clearly demonstrate the link between experimental observations and detailed mechanism as following: "To validate our above hypothesis, we performed isotope experiments using 13 C1labeled glucose (Supplementary Fig. 37) and 13 C1-labeled gluconate ( Supplementary   Fig. 38) as the substrates. As shown in Supplementary Fig. 37, 13 C-labeled formate was produced when 13 C1-labeled glucose was used as the substrate, indicating that the aldehyde group in glucose was converted to formate. In addition, semiquantitative 1H NMR analysis shows that the ratio of 13 C/ 12 C in the generated formate from 13 C1labeled glucose reached 0.3 (higher than the theoretical value of 0.2) at low charge, and it successively decreased at longer reaction time ( Supplementary Fig. 39). This trend can be explained by that the C1−C2 bond cleavage in glucose is more favourable, thus the 13 C1-labeled aldehyde group was firstly converted to formate. This is in well agreement with our previous theoretical calculation that glucose electrooxidation is initiated at C1−C2 position to give arabinose and formate, owing to the smallest bond order among the five C−C bonds 9 ." "In contrast, the ratio of 13 C/ 12 C in the generated formate from 13 C1-labeled gluconate is about 0.01 (equal to the natural abundance of 13 C/ 12 C) in the whole electrolysis process ( Supplementary Fig. 39). This result reveals that the acid group of gluconate cannot be converted to formate, thus route II can be excluded. Collectively, these results further demonstrated that the aldehydes, rather than acids, are the real intermediates of  Table 4)." Considering 100% of faradaic efficiency, I think informing the green hydrogen productivity is also important.

Response:
We agree with the reviewer. The productivity of hydrogen was calculated to be 0.56 g h -1 at a current of 15 A with near 100% Faradaic efficiency. The manuscript has been revised to highlight the importance of co-production of green hydrogen: "As a result, formate solution (562.8 mM) with space-time-yield (STY) of 256.6 mmol h−1 (corresponding to 11.8 g h −1 ) and H2 (>99.9% purity) with STY of 279.8 mmol h−1 (corresponding to 0.56 g h −1 ) were continuously produced using this stacked SPCFR system (Supplementary Figs. 43,44), achieving co-production of biomass-derived valuable chemicals and H2 fuel." (Please see Page 15 in the revised Manuscript) Comment 5: In the large-scale electrosynthesis experiments, the authors state that the cell working temperature is around 70 °C and that they must work on temperature management. I think that instead of cooling down the system, it is maybe better to optimize the method at this relatively high temperature.
Response: This is a very constructive comment. Indeed, it is maybe better to operate the electrolysis (e.g., traditional water splitting) at relatively high temperature, especially for improving energy efficiency. However, a higher electrolyte temperature may accelerate the non-Faradaic degradation rate of unstable biomass platforms, resulting in lower selectivity to targeted electrocatalytic product.
To estimate the temperature effect on non-Faradaic degradation, we conducted glucose degradation reaction in 1 M KOH at 70 °C without applying electrolysis. As shown in revised Supplementary Fig. 48a, more than 80% of glucose was consumed within 5 min, corresponding to a reaction rate of 2.68×10 −4 mol L −1 s −1 . Finally, glucose was completely degraded within 30 min, mainly delivering lactate (46.3% yield) and humins as dark-brown pigments (revised Supplementary Fig. 48b, c). In contrast, at room temperature (25 °C), only 41.4% of glucose was consumed after 7-h reaction ( Supplementary Fig. 12a), giving a much lower degradation rate (1.64×10 −6 mol L−1 s −1 ).
These results suggest that high temperature may dramatically increase non-Faradaic degradation rate by two orders of magnitude (specifically, 163 times shown in the revised Supplementary Fig. 48b), and result in low formate selectivity (<65%) for reactions without temperature management (entries 4−6 of Supplementary Table 5).
Therefore, we recommended to manage the temperature of electrolyte to suppress degradation of unstable biomass derivatives during electrolysis, thereby improving the selectivity of targeted product via electrolytic transformation.
Furthermore, we agree with the reviewer that it is better to optimize the method at relatively high temperature for electrolyzing stable substrates, such as bio-ethanol, which is beneficial for lowering electricity consumption of H2 production. (Nat.